Synthesis and characterisation of magnesium methyl complexes with monoanionic chelating nitrogen donor ligands and their reaction with dioxygen

Article abstract of DOI:10.1039/b001316l

Treatment of the Grignard reagent MeMgCl with the lithiates Li[L-X] (Li[L-X] = lithium ss-diketiminate [HC{C(Me)=NAr'}2Li] (Ar' =: 2,6-diisopropyIphenyl or lithium W.W-diisopropylaminotroponiminate, Li[('Pr2)ATI] in THF provided four-co-ordinate methylmag

Full text of DOI:10.1039/b001316l

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Synthesis and characterisation of magnesium methyl complexes
with monoanionic chelating nitrogen donor ligands and their
reaction with dioxygen
Philip J. Bailey,* Caroline M. E. Dick, Sylvie Fabre and Simon Parsons
Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh,
UK EH9 3JJ. E-mail: Philip.Bailey@ed.ac.uk
Received 17th February 2000, Accepted 3rd April 2000
Published on the Web 27th April 2000
Treatment of the Grignard reagent MeMgCl with the lithiates Li[L–X] (Li[L–X] = lithium β-diketiminate
[HC{C(Me)᎐NArЈ} Li] (ArЈ = 2,6-diisopropylphenyl) or lithium N,NЈ-diisopropylaminotroponiminate,
᎐
2
Li[(iPr2)ATI]) in THF provided four-co-ordinate methylmagnesium complexes [Mg(η2-L–X)Me(THF)]. The
β-diketiminate complex has been characterised by X-ray crystallography, however the aminotroponiminate
complex is an oil. Both complexes readily react with oxygen to provide methoxide-bridged dimeric complexes
[Mg(µ-OMe)(η2-L–X)]2 and the complex [Mg(µ-OMe){η2-(iPr2)ATI}]2 has structurally been characterised. The
methyl-bridged dimeric complex [Mg(µ-Me){HC[C(Me)NArЈ]}2]2 may be obtained by removal of THF from the
adduct under vacuum at 150 ЊC or by treatment of the β-diketimine (L–XH) with dimethylmagnesium in toluene
with elimination of methane, and has also been characterised crystallographically. In contrast to this, treatment
of MgMe2 with the aminotriponimine H[(iPr2)ATI] provides only the bis-chelate complex [Mg{(iPr2)ATI}2] which
has also been characterised structurally. However the methyl bridged dimer [Mg(µ-Me){η2-(iPr2)ATI}]2 may be
formed by removal of THF from [MgMe{η2-(iPr2)ATI}(THF)] at 110 ЊC under vacuum.
poniminato12,13 and iminoamide14 complexes and their cationic
Introduction
monomethyl derivatives. Although the activities of these
systems fall far short of those of the most active transition
metal catalysts,15 they illustrate the potential of main group
metal systems in a field previously considered to fall entirely
within the realm of d- and f-block metal chemistry. Given these
developments in aluminium chemistry, we have recently init-
iated a project to investigate the chemistry of nitrogen ligated
organomagnesium species and we report here our studies on
diketiminato and N,NЈ-dialkylaminotroponiminato complexes
derived from Grignard reagents and dimethylmagnesium, and
their reactivity with oxygen to provide methoxide species.
The anionic chelating nitrogen donor diketiminato A and
N,NЈ-dialkylaminotroponiminato (B, C) ligands have widely
been employed in transition metal chemistry where the steric
flexibility afforded by variation of the donor nitrogen sub-
stituents has extensively been exploited.1,2 However, the co-
ordination chemistry of these ligands with main group metals
has less extensively been explored. Given their extensive use as
reagents in both organic and organometallic synthesis, the
chemistry of organometallic derivatives of the s-block metals
has a special significance, however the complexity of their
solution chemistry often hinders investigation of their mode of
action.3 The complexation of the organomagnesium species
with polydentate nitrogen donor ligands has been a recurring
theme in this area and has led to a number of advances in
understanding of the structure, bonding and reactivity of the
Mg–C bond.4–8 The additional stability conferred by chelation
provides a relatively less labile, well defined, system whose
identity in solution may more confidently be equated to inform-
ation obtained from solid state structural studies, thus allowing
the development of structure–reactivity correlations. Further-
more, the electronic and steric properties of the ligand system
may be tailored to modify the processes occurring at the metal
centre and prevent unwanted reactions such as complex
dimerisation or bis-ligand complex formation, thus potentially
allowing the development of new modes of reactivity. These
considerations have elegantly been demonstrated in p-block
organometallic chemistry over recent years in the modification
of the well known ethene oligomerisation (aufbau) activity of
trialkylaluminium species AlR3. Following Jordan’s report of
the ability of cationic, three-co-ordinate methylaluminium
amidinato complexes (D) to polymerise ethene,9 the first
example of such activity for a main group metal complex, a
number of groups have reported their characterisation of
dimethylaluminium diketiminato,10,11 N,NЈ-dialkylaminotro-
Experimental
General procedures
2-(2,6-Diisopropylphenylamino)-4-(2,6-diisopropylphenyl-
imino)-2-pentene,1 ArЈN᎐C(Me)C(H)᎐C(Me)NHArЈ, and N-
᎐
᎐
isopropyl-2-(isopropylamino)troponimine12
[2-(isopropyl-
amino)-1-isopropyliminocyclohepta-2,4,6-triene]
H[(iPr)2-
ATI] were prepared according to the literature methods.
Methylmagnesium chloride (3 M solution in THF), n-butyl
DOI: 10.1039/b001316l
J. Chem. Soc., Dalton Trans., 2000, 1655–1661
This journal is © The Royal Society of Chemistry 2000
1655

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3
lithium (1.6 M solution in hexanes) and deuteriated NMR
solvents were purchased from Aldrich and used as received. All
reactions and manipulations were undertaken under an atmos-
phere of purified nitrogen in standard Schlenk apparatus or
inside a Saffron Scientific glove-box unless otherwise stated.
Diethyl ether and hydrocarbon solvents were distilled from
sodium–benzophenone under an atmosphere of nitrogen
immediately prior to use. Chlorinated solvents were distilled
from CaH2. All NMR spectra were recorded on a Bruker AC
250 spectrometer. Dimethylmagnesium was prepared by adding
1 equivalent of MeLi in ether to a THF solution of MeMgCl at
room temperature. After removing THF under reduced pres-
sure, the product was extracted from LiCl with ether. Ether
was removed under vacuum and the residue dried for 5 hours at
150 ЊC under vacuum. The product was isolated as a white solid
in 60% yield and analysed for chloride gravimetrically by
hydrolysis and treatment with silver nitrate. Typically the chlor-
ide content was found to be in the region of 5% by weight.
3JH-H = 6.0), 6.27 (t, 2 H, H4, JH-H = 10.0), 6.56 (d, 4 H, H2,6
,
3JH-H = 12.0) and 6.98 (dd, 4 H, H3,5, 3JH-H = 10.0 Hz). 13C-{1H}
NMR (C6D6): δ 24.0 (CH3), 47.9 (CH), 50.4 (CH3), 111.7 (C4),
115.3 (C2,6), 134.6 (C3,5), 162.6 (C1,7). Calc. for C14H22MgN2O:
C, 65.01; H, 8.57; N, 10.82. Found: C, 64.94; H, 7.73; N,
10.88%.
[Mg(ꢁ-Me){HC[C(Me)NArЈ]2}]2
4 (ArЈ ؍ 2,6-diisopropyl-
phenyl). Method a. To a stirred suspension of Me2Mg (0.38 g,
7.1 mmol) in 50 cm3 of toluene was added ArЈN᎐C(Me)C-
᎐
(H)᎐C(Me)NHArЈ (3 g, 7.1 mmol). The mixture was stirred for
᎐
2 days at room temperature. During this period MgMe2 was
slowly dissolved and reacted with the ligand. The product,
insoluble in toluene, was isolated by filtration as a white solid in
a 60% yield. A few X-ray quality crystals were obtained by
standing the product overnight in suspension in toluene.
Method b. White crystals of [MgMe{HC[C(Me)NArЈ]2}-
(THF)] 1 (1.05 g, 2.0 mmol) were heated under vacuum at
150 ЊC for 1 hour to afford compound 4 as a white solid in
quantitative yield (0.91 g, 1.0 mmol). 1H NMR (THF-d8):
δ Ϫ2.00 (s, 3 H, CH3), 1.15 (d, 24 H, CH3, 3JH-H = 6.0), 1.6 (s, 6
Preparations
[MgMe{HC[C(Me)NArЈ]2}(THF)]
1
(ArЈ ؍ 2,6-diiso-
3
H, CH3), 1.70 (m, 4 H, CH2), 3.20 (spt, 4 H, CH, JH-H = 7.0
propylphenyl). The compound n-BuLi (7.4 cm3, 1.6 M in hexane,
11.9 mmol) was added dropwise via syringe to a stirred solution
Hz), 3.71 (t, 4 H, CH2), 4.70 (s, 1 H, CH) and 6.9–7.1 (m, 6 H,
CH arom.). 13C-{1H} NMR (THF-d8): δ Ϫ20.5 (CH3), 23.1
(CH3), 22.3 and 24.3 (CH3), 24.1 (CH2), 27.4 (CH), 66.2 (CH2),
93.7 (Cβ), 122.8 (Cm), 124.0 (Cp), 141.8 (Co), 145.3 (Cipso) and
167.2 (Cα). Calc. for C30H44Mg2N2: C, 78.84; H, 9.70; N, 6.12.
Found : C, 77.64; H, 9.52; N, 6.10%.
of ArЈN᎐C(Me)C(H)᎐C(Me)NHArЈ (5 g, 11.9 mmol) in 100
᎐
᎐
cm3 of THF at Ϫ70 ЊC. The mixture was allowed to warm to
room temperature and stirred for 30 min. A THF solution of
MeMgCl (3.9 cm3, 3 M in THF, 11.9 mmol) was added drop-
wise at room temperature. The resulting mixture was stirred at
room temperature for 1 hour, the volatiles were removed under
vacuum and the product was extracted from the LiCl with 100
cm3 of hexane. White crystals of compound 1 deposited over-
night at Ϫ20 ЊC and were collected by filtration (4.9 g, 78%). 1H
NMR (CDCl3): δ Ϫ2.00 (s, 3 H, CH3), 1.10 (d, 24 H, CH3,
3JH-H = 6.0), 1.6 (s, 6 H, CH3), 1.82 (t, 4 H, CH2), 3.11 (spt, 4 H,
[Mg{ꢀ2-(iPr)2ATI}]2 5. To a stirred suspension of MgMe2
(0.13 g, 2.4 mmol) in 20 cm3 of toluene was added H[(iPr)2ATI]
(0.5 g, 2.4 mmol). The mixture was stirred for 12 h. The toluene
was removed under vacuum and the product extracted from the
excess of MgMe2 with 20 cm3 of hexane. The extract was evap-
orated to dryness under vacuum yielding [Mg{η2-(iPr)2ATI}2]
as a yellow solid in 98% yield (0.5 g) based upon H[(iPr)2ATI]
added. Suitable crystals for X-ray diffraction analysis were
obtained from a saturated hexane solution at Ϫ20 ЊC. 1H NMR
3
CH, JH-H = 7.0 Hz), 3.71 (t, 4 H, CH2), 4.71 (s, 1 H, CH) and
6.8–7.2 (m, 6 H, CH arom.). 13C-{1H} NMR (CDCl3): δ Ϫ19.8
(CH3), 23.8 (CH3), 24.0 and 24.8 (CH3), 28.1 (CH2), 27.6 (CH),
69.4 (CH2), 93.7 (Cβ), 123.1 (Cm), 124.2 (Cp), 142.1 (Co), 145.3
(Cipso) and167.6 (Cα). Calc. for C34H52MgN2O: C, 77.18; H,
9.90; N, 5.29. Found: C, 77.24; H, 10.16; N, 5.09%.
3
(C6D6): δ 0.89 (d, 12 H, CH3, JH-H = 6.4), 0.90 (d, 12 H, CH3,
3JH-H = 6.2), 3.47 (spt, 4 H, CH, JH-H = 6.0), 5.97 (t, 2 H, H4,
3
3JH-H = 8.8), 6.26 (d, 4 H, H2,6, JH-H = 10.6) and 6.66 (dd, 4 H,
3
H3,5, 3JH-H = 10.8 Hz). 13C-{1H} NMR (C6D6): δ 24.1 (CH3) 48.3
(CH), 113.1 (C2,6), 116.1 (C4), 135.0 (C3,5) and 162.9 (C1,7). Calc.
for C26H38MgN4: C, 72.47; H, 8.88; N, 12.99. Found: C, 72.18;
H, 8.98; N, 12.61%.
[MgMe{ꢀ2-(iPr)2ATI}(THF)] 2. The compound n-BuLi (1.5
cm3, 1.6 M in hexane, 2.4 mmol) was added dropwise via
syringe to a stirred solution of H[(iPr)2ATI] (0.5 g, 2.4 mmol) in
THF at 0 ЊC. After stirring for 30 min, the mixture was allowed
to warm to room temperature and stirred for 30 min. A THF
solution of MeMgCl (0.8 cm3, 3 M in THF, 2.4 mmol) was
added dropwise at room temperature and the resulting mixture
stirred for 30 min. The volatiles were removed under vacuum
and the product was extracted from the LiCl with 20 cm3 of
hexane. Hexane was removed under vacuum, yielding the crude
product as an orange oil in 98% yield (0.74 g). 1H NMR (C6D6):
δ Ϫ0.95 (s, 3 H, CH3), 0.99 (t, 4 H, CH2), 1.09 (d, 12 H, CH3,
3JH-H = 6.0), 3.16 (t, 4 H, CH2), 3.65 (spt, 2 H, CH, 3JH-H = 6.4),
[MgMe({ꢀ2-(iPr)2ATI}]2 6. The orange oil [MgMe{η2-
(iPr)2ATI}(THF)] 2 (0.60 g, 1.91 mmol) was heated under
vacuum at 110 ЊC for 1 hour to afford compound 6 as an orange
1
powder in quantitative yield (0.46 g, 0.95 mmol). H NMR
(C6D6): δ Ϫ0.61 (s, 3 H, CH3), 1.14 (d, 12 H, CH3, 3JH-H = 6.0),
3.71 (spt, 2 H, CH, JH-H = 6.0), 6.23 (t, 1 H, H4, JH-H = 9.0),
6.50 (d, 2 H, H2,6, 3JH-H = 11) and 6.90 (dd, 2 H, H3,5, 3JH-H = 10,
11 Hz). 13C-{1H} NMR (C6D6): δ Ϫ11.2 (CH3), 23.9 (CH3), 48.0
(CH), 112.9 (C2,6), 115.9 (C4), 134.8 (C3,5) and 162.7 (C1,7).
3
3
3
3
6.1 (t, 1 H, H4, JH-H = 9.0), 6.35 (d, 2 H, H2,6, JH-H = 11) and
6.82 (dd, 2 H, H3,5, JH-H = 10, JH-H = 11 Hz). 13C-{1H} NMR
(C6D6): δ Ϫ14.2 (CH3), 23.9 (CH3), 25.1 (CH2), 47.9 (CH), 68.6
(CH2), 110.7 (C2,6), 114.5 (C4), 134.4 (C3,5) and 162.4 (C1,7).
Calc. for C18H30MgN2O: C, 68.69; H, 9.60; N, 8.89. Found: C,
67.86; H, 9.79; N, 8.99%.
3
3
Crystal structure solution and refinement for compounds 1, 3, 4
and 5
All data sets were collected at 220 K using Cu-Kα radiation
on a Stoe Stadi-4 diffractometer equipped with an Oxford
Cryosystems low-temperature device. Absorption corrections
were performed using ψ-scan data for compounds 1 and 3;
corrections for 4 and 5 were carried out using Gaussian integr-
ation following refinement of the crystal face indices and
dimensions against a set of ψ scans (Stoe X-Shape).16 The struc-
tures were solved by direct methods (SHELX 97 or SIR 92)17
and refined against F2 using SHELX 97.17 Hydrogen atoms
were placed in calculated positions. In 1 the THF is disordered
in the ratio 50:50 over two conformations with a common
oxygen-position; the hexane of solvation is also disordered over
[(Mg(OMe){ꢀ2-(iPr)2ATI}]2 3.
A
stirred solution of
[MgMe{η2-(iPr)2ATI}(THF)] 2 (0.75 g, 2.4 mmol) in 20 cm3 of
hexane was treated with pure O2 by bubbling the gas through it
for 5 min. A yellow solid was obtained in suspension which was
isolated by filtration and dried under vacuum (0.33 g, 53%).
Suitable crystals for X-ray diffraction analysis were obtained
from a saturated hexane solution of compound 2 in the
presence of traces of air at Ϫ20 ЊC. 1H NMR (C6D6): δ 1.32 (d,
24 H, CH3, 3JH-H = 6.0), 3.24 (s, 6 H, CH3), 3.82 (spt, 4 H, CH,
1656
J. Chem. Soc., Dalton Trans., 2000, 1655–1661

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Table 1 Crystal data and results of the structure analyses of compounds 1, 3, 4 and 5
1
3
4
5
Crystal description
Empirical formula
Colourless plate
C37H59MgN2O
[Mg(L–X)Me(THF)]ؒ0.5C6H14
572.17
Orange block
C28H44Mg2N4O2
[Mg(L–X)(OMe)]2
517.29
Colourless block
C60H88Mg2N4
[Mg2(L–X)2(µ-Me)2]
913.96
Yellow block
C26H38MgN4
[Mg(L–X)2]
430.91
Mw
Crystal system
Space group
Triclinic
P1
Orthorhombic
Pbca
Monoclinic
P21/n
Monoclinic
P21/n
¯
a/Å
b/Å
c/Å
α/Њ
9.136(3)
12.690(5)
16.004(6)
99.00(3)
96.17(3)
95.19(2)
1811.2(11)
2
0.621
5022
2393
0.0840
0.2445
15.560(11)
8.838(7)
22.162(16)
14.068(5)
14.903(7)
14.110(5)
9.5281(6)
18.3692(13)
14.2891(10)
β/Њ
108.02(3)
93.154(7)
γ/Њ
V/Å3
3048(4)
4
0.927
2715
1631
0.0582
0.1871
2813(2)
2
0.666
4807
3228
0.0720
0.2189
2497.1(3)
4
0.747
4652
4010
0.0346
0.0931
Z
µ(Cu-Kα)/mmϪ1
Independent reflections
Data with I > 2σ(I)
R1
wR2
Table 2 Selected bond distances (Å) and angles(Њ) for compounds 1, 3, 4 and 5
1
3 (# = Ϫx ϩ 2, Ϫy ϩ 1, Ϫz ϩ 1)
4 (# = Ϫx ϩ 1, Ϫy, Ϫz)
5
Mg–N(1)
Mg–N(5)
Mg–O(1T)
Mg–C(1M)
N(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–N(5)
2.061(5) Mg–N(1) 2.034(3) Mg–N(1)
2.075(3) Mg–N(1A)
2.076(3) Mg–N(1B)
2.296(3) Mg–N(2A)
2.0528(11)
2.063(5) Mg–N(2)
2.066(4) Mg–O(1M)
2.107(6) Mg–O(1M)#
1.344(6) N(1)–C(11)
1.396(7) N(2)–C(2)
1.410(7) C(1)–C(2)
1.341(6) C(2)–C(3)
C(3)–C(4)
105.12(16) C(4)–C(5)
104.74(16) C(5)–C(6)
2.046(3) Mg–N(5)
1.941(3) Mg–C(1M)
1.932(3) Mg–C(1M)#
1.516(6) N(1)–C(2)
1.321(4) C(2)–C(3)
1.506(5) C(3)–C(4)
1.401(4) C(4)–N(5)
1.373(5) N(1)–Mg–N(5)
1.373(6) C(1M)–Mg–C(1M)#
1.373(6) Mg#–C(1M)–Mg
1.385(5) Mg–N(1)–C(2)
78.94(12) Mg–N(1)–C(11)
81.88(12) C(2)–N(1)–C(11)
126.92(13) Mg–N(5)–C(4)
122.90(14) Mg–N(5)–C(51)
131.26(13) C(4)–N(5)–C(51)
121.15(12)
2.0502(11)
2.0461(12)
2.0420(12)
1.3294(17)
1.3268(17)
1.3276(17)
1.3325(17)
1.4997(18)
1.4176(19)
1.386(2)
1.367(3)
1.382(3)
1.378(2)
1.5002(18)
1.4175(19)
1.379(2)
1.371(2)
1.370(2)
2.259(3) Mg–N(2B)
1.315(5) N(1A)–C(1A)
1.410(5) N(2A)–C(2A)
1.397(4) N(1B)–C(1B)
1.332(4) N(2B)–C(2B)
92.39(10) C(1A)–C(2A)
102.80(11) C(2A)–C(3A)
77.20(11) C(3A)–C(4A)
C(1M)–Mg–O(1T) 107.3(2)
N(1)–Mg–O(1T)
N(5)–Mg–O(1T)
N(1)–Mg–C(1M)
N(5)–Mg–N(1M)
N(1)–Mg–N(5)
Mg–N(1)–C(2)
Mg–N(1)–C(1A)
C(1A)–N(1)–C(2)
Mg–N(5)–C(4)
124.7(2)
119.5(2)
C(6)–C(7)
N(1)–Mg–N(2)
121.3(2)
123.3(2)
115.4(3)
120.5(2)
123.1(2)
116.4(3)
C(4A)–C(5A)
C(5A)–C(6A)
C(6A)–C(7A)
C(1B)–C(2B)
C(2B)–C(3B)
C(3B)–C(4B)
C(4B)–C(5B)
C(5B)–C(6B)
C(6B)–C(7B)
93.09(18) O(1M)#–Mg–O(1M)
121.1(3)
121.1(3)
117.7(4)
121.8(3)
118.6(3)
119.6(4)
O(1M)#–Mg–N(1)
O(1M)–Mg–N(1)
O(1M)#–Mg–N(2)
O(1M)–Mg–N(2)
Mg–N(1)–C(1)
Mg–N(1)–C(11)
C(1)–N(1)–C(11)
Mg–N(2)–C(2)
Mg–N(5)–C(1B)
C(1B)–N(5)–C(4)
116.0(2)
124.9(2)
1.381(2)
118.1(3)
115.4(2)
N(1A)–Mg–N(2A) 79.02(4)
N(1B)–Mg–N(2B) 79.37(4)
Mg–N(2)–C(21)
C(2)–N(2)–C(21)
123.5(2)
121.0(3)
two conformations about a crystallographic inversion centre.
Similarity restraints were applied to chemically equivalent
bond lengths and angles involving partial-weight atoms. All
full-weight non-H atoms were refined with anisotropic dis-
placement parameters (adps) with similarity restraints applied
component; it was possible to refine isotropic displacement
parameters for all atoms. The major component was refined
normally with adps for all atoms. Refinement of 5 presented no
problems, and all non-H atoms were refined with adps.
Crystallographic details are provided in Table 1 and selected
bond distances and angles in Table 2.
i
globally for light atoms. In 3 one Pr group is orientationally
disordered in the ratio 75:25; similarity restraints were again
applied to bond lengths and angles in the disordered region. All
non-H atoms except those of the minor disorder component
were refined with adps. In 4 the whole complex appears to be
disordered over two orientations in the ratio 93:7 (this ratio
was refined as part of the model). It is clear from the cell dimen-
sions that this structure could be twinned via a twofold rotation
about [101], but an attempt to refine it as a twin yielded a twin-
component scale factor of 0.01, suggesting that this was not the
source of the apparent disorder. The real source of this image
remains unclear and we speculate that it may be due to the
presence of a supercell (although we have no evidence of this
from the initial reflection search); for the purposes of determin-
ing connectivity though it is sufficient to treat this effect as
disorder. During refinement the minor component was con-
strained to be a rigid body with the same geometry as the major
CCDC reference number 186/1919.
See http://www.rsc.org/suppdata/dt/b0/b001316l/ for crystal-
lographic files in .cif format.
Results and discussion
Treatment of the lithium β-diketiminate [HC{C(CH )᎐NArЈ} -
᎐
3
2
Li] (ArЈ = 2,6-diisopropylphenyl)1,10 with MeMgCl in THF
provides the THF solvated methyl magnesium complex
[MgMe{HC[C(Me)NArЈ]2}(THF)] 1 which may be isolated
as colourless crystals from hexane solution in 78% yield
1
(Scheme 1). The H NMR spectrum of 1 in CDCl3 shows a
characteristic signal at δ Ϫ2.0 attributable to the magnesium
bound methyl and a corresponding signal at Ϫ19.8 in the
13C NMR spectrum is also observed. The occurrence of only
1
one set of signals due to the nitrogen substituents in both H
J. Chem. Soc., Dalton Trans., 2000, 1655–1661
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Fig. 2 Molecular structure of [Mg(µ-OMe){η2-(iPr2)ATI}]2 3.
Fig. 1 Molecular structure of [MgMe{η2-HC[C(Me)NArЈ]2}(THF)]
1 (ArЈ = 2,6-diisopropylphenyl).
(iPr2)ATI}(THF)] 2 as an orange oil in quantitative yield after
extraction from LiCl into hexane (Scheme 1). In C6D6 solution
the methyl ligand in this complex gives rise to signals at δ Ϫ0.95
1
and Ϫ14.2 in the H and 13C NMR spectra respectively, and
signals for co-ordinated THF are also clearly observed. The
corresponding dimethylaluminium complex of this ligand has
also previously been characterised,12 although, as far as we
are aware, this is the first example of a magnesium N,NЈ-
dialkyltroponiminato complex. Attempted crystallisation of
2 from hexane provided an orange crystal, however X-ray
analysis showed this not to be the methyl complex, but rather
the methoxy-bridged dimer [Mg(µ-OMe){η2-(iPr2)ATI}]2
3
(Fig. 2). The Mg–O bond distances in the centrosymmetric
dimer do not differ significantly and the nitrogen and ring
atoms are coplanar indicating effective delocalisation in this
system. The smaller bite angle of this five membered chelate
ring is clearly evident in the narrow N–Mg–N angle of
78.94(12)Њ. It is interesting that, despite this, the Mg–N dis-
tances [2.034(3) and 2.046(3) Å] are slightly shorter than those
in 1 possibly indicating the increased donor properties of the
(iPr2)ATI ligand attributable to the potential for a tropylium/
diamide type charge separation in this ligand (C), although
structural comparison with 2 would be necessary for firm
conclusions to be drawn. Unfortunately we have so far been
unsuccessful in obtaining crystals of 2. However, the observ-
ation of similar differences in metal–nitrogen bond distances
for the dimethylaluminium complexes of these two ligands
supports this hypothesis.10,12
Scheme 1 Synthesis of complexes 1 and 2. Reagents and conditions: (i)
n-BuLi, THF, Ϫ78 ЊC, 1 h; (ii) MeMgCl, THF, 25 ЊC, 30 min.
and 13C NMR spectra indicates the operation of a dynamic
process; this is likely to be the reversible dissociation of the
THF molecule rather than rotation about the C–N bonds given
the steric congestion provided by the 2,6-diisopropylphenyl
groups. Such a process is consistent with the observation that
the THF may be removed from 1 under vacuum, as discussed
below, indicating a relatively labile Mg–O bond. The crystal
structure of 1 (Fig. 1) shows the complex to adopt a distorted
tetrahedral co-ordination geometry around magnesium. The
N–Mg–N chelate bite angle is 93.09(18)Њ and the six-membered
chelate ring deviates from planarity to a considerable degree.
This ring is best described as a distorted cyclohexane boat
structure in which the magnesium and central carbon (C3)
atoms lie 0.1256(27) and 0.2675(60) Å above the least squares
plane, while N1 and N5 lie 0.1370(27) and 0.1225(27) Å below.
This contrasts with the structure of the Me2Al complex of the
same ligand in which the N–Al–N angle is 96.18Њ and the five
ligand atoms are essentially coplanar; the aluminium atom is
displaced by 0.72 Å from this plane.10 The differing sizes of Mg
and Al are reflected in the metal–carbon bond distance of
2.107(6) Å in 1 which compares with 1.958(3) and 1.970(3) Å in
the aluminium complex. A number of other nitrogen ligated
methyl magnesium complexes have been structurally character-
ised and these include [MgMe2(Me2NCH2CH2NMe2)] (Mg–C
2.166(6) Å),5 [MgMe{Me2N(CH2)2NMe}]2 (Mg–C 2.100(4)
Å),6 [MgMe2(C7H13N)2] (Mg–C 2.163(9) and 2.224(8) Å;
C7H13N = quinuclidine),7 and [MgMe{HB(pz)3}] (Mg–C
2.118(11) Å).8 The only other structurally characterised mag-
nesium β-diketiminato complex is the bis-chelate [Mg{η2-
HC(CPhNSiMe3)2}2] which also displays distorted tetrahedral
co-ordination geometry around magnesium with N–Mg–N
angles of 99.7(3) and 114.3(3)Њ.18 Interestingly, although the
Mg–N bond distances do not differ significantly from those in
1, the six membered chelate rings are found to be planar in this
structure.
Our initial concern that the conversion of compound 2 into 3
had resulted from contamination with methanol during crystal-
lisation was ruled out when it was observed that controlled
exposure of 2 in d8-THF solution to dry air or pure oxygen led
1
to the disappearance of the methyl ligand H NMR signal at
δ Ϫ0.95 and the appearance of the methoxide signal of 3 at
δ 3.24. The intermediacy of a further species in this reaction,
which we tentatively assign as a methyl peroxide complex, may
be inferred from the initial rapid transformation to a species
1
with a H NMR signal at δ 3.40 on treatment of 2 with O2,
followed by the slower (ca. 5 min) decay of this signal concom-
itant with the growth of that due to the methoxide (Scheme 2).
The nature of this intermediate, its molecularity and the co-
ordination mode of the methyl peroxide is of course open
to speculation as the structural characterisation of an alkyl
peroxide complex of magnesium has never been reported. The
complex 3 could be isolated in 53% yield as an orange powder
from this reaction with oxygen. Following this observation we
found that similar treatment of 1 also provides a methoxide
complex, as indicated by a similar replacement of the methyl
ligand signal with one due to methoxide at δ 3.21, although in
this case the conversion is more rapid and no intermediate
species could be observed. However, in this case we have been
unable to isolate the methoxide complex as the oxidation
appears to proceed beyond this stage for 1 and only a mixture
of decomposition products has been obtained. This behavior is
in contrast to that of the corresponding dimethylaluminium
Treatment of lithium N,NЈ-diisopropylaminotroponiminate,
Li[(iPr2)ATI],12 with MeMgCl in THF provides [MgMe{η2-
1658
J. Chem. Soc., Dalton Trans., 2000, 1655–1661

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Scheme 2 Reaction of compound 2 with oxygen. Reagents and con-
ditions: (i) O2 gas, d8-THF, <1 min; (ii) second mol of 2, ca. 5 min.
complex which ‘can be handled in air without decomposition’,10
but is consistent with the observed formation of alkyl peroxide
complexes on reaction of hydrotris(pyrazolyl)boratomag-
nesium alkyl complexes with oxygen.19 The reaction of
Grignard reagents with oxygen has long been known as a route
to hydroperoxides and alcohols,20 and there is both direct and
indirect evidence that the reaction proceeds via a radical mech-
anism.21 The only previously reported example of a structurally
characterised magnesium alkoxide complex formed by oxygen
insertion into a Mg–C bond is one derived from the hetero-
metallic system [{Me2Al(µ-iPr2N)2Mg(µ-Me)}4] which provides
a dimeric methoxide bridged system analogous to 3 but with
[η2-Me2Al(NiPr2)2]Ϫ ligands.22 The structures of cyclic oxo- and
peroxo-centred heterometallic Li/Mg and Na/Mg amides
derived by exposure of the precursors to oxygen have also
recently been reported.23
Scheme 3 Synthesis of complexes 4, 5 and 6. Reagents and conditions:
(i) MgMe2, toluene, ϪCH4; (ii) MgMe2, THF, ϪCH4; (iii) 150 ЊC,
One of our aims in developing this chemistry is the form-
ation of three-co-ordinate magnesium methyl complexes;
neutral analogs of the ethene polymerisation active, cationic
aluminium amidinate species produced by Jordan (D).9 In order
to allow access to the catalytic site by the incoming ethene, the
co-ordination environment of the metal in such three-co-
ordinate species is required to be somewhat open, and the metal
may be regarded as possessing a vacant site. In the active
aluminium amidinate species this site has been shown to be
weakly ligated by the methyl adduct of the Lewis acid used to
activate the precatalyst, and such weak associations have now
been established crystallographically in many active, single site
catalyst species.24 For our target neutral magnesium species,
such Lewis acid activation would be unnecessary and con-
sequently, without the specific introduction of a weakly co-
ordinating agent, none is available to occupy the vacancy.
Strongly co-ordinating solvents, and in particular ethers,
occupy the vacant site to provide complexes of the type 1 and 2
which would not be expected to show activity towards ethene.
We have consequently sought to produce ether free species and
have found that, contrary to expectation, it is possible to
remove the co-ordinated THF from complex 1 by heating to
150 ЊC under vacuum (Scheme 3). The resulting methyl bridged
dimer [Mg(µ-Me){HC[C(Me)NArЈ]2}]2 4 shows only minimum
solubility in hydrocarbon (hexane, toluene, benzene) solvents
and NMR spectra were therefore unobtainable, however crys-
tals suitable for X-ray analysis were obtained by allowing a
suspension of 4 in toluene to stand overnight. The NMR spec-
tra in d8-THF indicate that the dimer is cleaved by this solvent
to provide d8-THF–1.
vacuum; (iv) H[(iPr2)ATI], ϪCH4; (v) 110 ЊC, vacuum.
Fig. 3 Molecular structure of [Mg(µ-Me){HC[C(CH3)NArЈ]2}]2
(ArЈ = 2,6-diisopropylphenyl).
4
X)MgMe] via the formation of methyl bridges creates a highly
crowded molecule, it is clear that the 2,6-diisopropylphenyl
ligand substituents are insufficiently bulky to prevent this
from occurring. The crystal structure of 4 shows the complex to
be centrosymmetric with the four-co-ordinate magnesium ions
in a distorted tetrahedral environment imposed by the narrow
ligand chelate (N–Mg–N) angle of 92.39(10)Њ, a feature also
noted in the structure of 1. The ArЈN᎐C(Me)-C(H)᎐C(Me)-
᎐
᎐
The molecular structure of compound 4 is shown in Fig. 3.
NArЈ backbone of the diketiminato ligand is not planar but
adopts a boat conformation similar to that observed for 1. The
Although the dimerisation of the three-co-ordinate [(η2-L–
J. Chem. Soc., Dalton Trans., 2000, 1655–1661
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origin cannot be ruled out. The N–Mg–N angles in 5 [79.37(4)
and 79.02(4)Њ] are narrower than those observed in [AlMe2-
{(iPr2)ATI}] (83.3Њ), and the four Mg–N bond lengths
[2.0528(11), 2.0461(12), 2.0420(14) and 2.0502(11) Å] are
longer than in the AlMe2 complex as would be anticipated for
the larger metal. The N–C [range 1.3268(17) to 1.3325(17) Å]
and C–C [range 1.367(3) to 1.386(2) Å] bond lengths lie
between those for single and double N–C and C–C bonds which
shows that there is a degree of π delocalisation as with com-
i
plexes 1 and 3. The observation of two doublets for the Pr
1
groups at δ 0.89 and 0.90 in the H NMR spectrum of 5 indi-
i
cates the congestion which the crowding of four Pr groups
around the metal creates in the molecule, and suggests that the
inequivalence of the ligands observed crystallographically
persists in solution.
Fig. 4 Molecular structure of [Mg{η2-(iPr2)ATI}2] 5.
Following the observation that THF could be removed from
[MgMe{HC[C(Me)NArЈ]2}(THF)] 1 to provide the methyl
bridged dimer 4 by heating under vacuum, we found that the
putative dimer [Mg(µ-Me){(iPr2)ATI}]2 6 could also be formed
as an orange powder by similar treatment of the orange oil
[MgMe{(iPr2)ATI}(THF)] 2 at 110 ЊC (Scheme 3). The NMR
spectra of 6 indicate that all THF has been removed by the
thermolysis treatment, and the methyl ligand signal appears at
N–C and C–C bond lengths lie between those for single and
double N–C and C–C bonds indicating a degree of π delocalis-
ation. The Mg–Me distances in 4 [2.259(3) and 2.296(3) Å] are
longer than that found in 1 [2.107(6) Å] as would be anticipated
for a comparison of terminal and µ-co-ordinated methyl lig-
ands. However, despite the apparent steric crowding resulting
from the formation of the dimer, these Mg–C distances lie in
the range found in the three other neutral Mg(µ-Me)2Mg sys-
tems which have been structurally characterised to date. Thus,
Mg–C distances of 2.243/2.297, 2.270/2.284 and 2.200/2.277 Å
are found in the centrosymmetric systems [{η2-Me2Al(Et2N)-
Mg(µ-Me)}2],25 [{CpЈ(THF)Mg(µ-Me)}2]26 (CpЈ = C5Me4Et)
1
δ Ϫ0.61 and Ϫ11.2 in the H and 13C spectra respectively (cf.
Ϫ0.95 and Ϫ14.2 for the monomeric THF complex 2). On the
basis that 4, which has more sterically demanding nitrogen
substituents, exists as a methyl bridged dimer, we assign a
similar structure to 6, although in the absence of a structural
characterisation this cannot be conclusively proved. The
relatively facile removal of THF from these complexes indicates
a surprising level of lability, however this is not inconsistent
with the well established Schlenk equilibrium28 in which co-
ordinated ether must be displaced in the formation of alkyl
bridged intermediates which facilitate transfer of alkyl groups
between magnesium ions to form dialkylmagnesium species in
ether solutions, eqn. (1).
27
and [MeSi(tBuNAlMe2)(tBuNH){(tBuNMg(µ-Me)}]2 respec-
tively.
An alternative synthesis of compound 4 involves treatment
of the β-diketimine, which exists as the iminoamine tautomer
ArЈN᎐C(Me)C(H)᎐C(Me)NHArЈ, with one molar equivalent
᎐
᎐
of dimethylmagnesium in toluene, providing 4 in 60% yield with
liberation of methane (Scheme 3). The formation of 4 by
this route appears to be aided by its insolubility, and this is
illustrated by the exclusive formation of the toluene soluble
bis-chelate complex [Mg{η2-(iPr2)ATI}2], 5 in 98% yield when
the analogous reaction with the aminotroponimine is carried
out. The formation of 5 in this reaction can be accounted for by
both the greater toluene solubility, and therefore reactivity, of
the initially formed [Mg(µ-Me){η2-(iPr2)ATI}]2 6, which we
assume also to be a dimer, and the greater basicity of the methyl
ligands in this species than those in MgMe2. Thus, following
initial formation of 6 in solution, a rapid reaction with a second
aminotroponimine molecule ensues and all ligand is therefore
consumed by one half of the essentially insoluble MgMe2.
Although disproportionation of 6 into 5 and MgMe2 is a
possible alternative explanation, we discount this on the
grounds that such a process has not been observed for the
β-diketiminate complexes 1 and 4 and the ligand exchange
involved should be disfavored by the chelate effect. In THF, in
which MgMe2 is soluble, the anticipated formation of the THF
adducts 1 and 2 take place (Scheme 3).
X-Ray quality crystals of compound 5 were grown from
hexane solution at Ϫ20 ЊC and the resulting crystallographic
analysis (Fig. 4) shows the magnesium atom to be in a distorted
tetrahedral environment dictated by the narrow 79Њ N–Mg–N
angle in the 5-membered chelate rings. However, the bulk of the
iPr substituents ensures that the two N–Mg–N planes are
orthogonal, intersecting at an angle of 92.7Њ. On comparing
the structure of 5 with that of the aluminium complex
[AlMe2{(iPr2)ATI}],12 a number of differences are apparent.
The heterobicyclic ring system containing the 7-membered ring,
the two nitrogen atoms and the metal is planar in the alu-
minium complex, but in 5 there is significant distortion with the
N–Mg–N and N–C–C–N planes intersecting at 15.6 and 3.2Њ
for ligands A and B respectively. Whether this difference is
steric or electronic in origin is a moot point, however as the
comparison is between [LAlMe2] and [L2Mg] complexes a steric
(1)
Conclusion
It is clear that the ligand steric properties required to provide a
three-co-ordinate methylmagnesium species and prevent dimer
formation are subtle, as has previously been indicated for
aluminium by Jordan and co-workers.9 Whether such species
prove to be active for alkene polymerisation remains to be seen,
but they remain our target as potential first generation mag-
nesium based catalysts. Further studies will be directed at
modification of the ligand and magnesium co-ordinated alkyl
group properties to achieve this aim. Our investigations into
the reactivity of compounds 1, 2, 4 and 6 with other small
molecules are currently underway.
Acknowledgements
We are grateful to the EPSRC and Montell Polyolefins for
funding and Dr Andrew Horton (Shell Research and Technol-
ogy Centre Amsterdam) for helpful discussions.
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